I've been trying to figure out why validation errors are not shown reactively (they are detected) and I think the error is coming from simple schema's tracker integration with autoform. This is from a debugging session and I've not been able to verify by running a local copy so apologies if it's incorrect.

As a simple example, consider:

subSchema = {
     bar: {type: String} 
}

mainSchema = {
     foo: {type: subSchema}
}

Assuming mainSchema is the schema on the form but bar is empty, bar should set the class has-error and set the help-block. The problem I believe is that the code in ValidationContext does not trigger the tracker dependency to change.
ValidationContext._schemaKeys = ["foo"];
So when markKeyChanged is called with "foo.bar" , hasOwnProperty will return false and not trigger a change.

This is the code:

export default class ValidationContext {
  constructor(ss) {
    this._simpleSchema = ss;
    this._schema = ss.schema();
    this._schemaKeys = Object.keys(this._schema);
    this._validationErrors = [];
    // Set up validation dependencies
    this._deps = {};
    const { tracker } = ss._constructorOptions;
    if (tracker) {
      this._depsAny = new tracker.Dependency();
      this._schemaKeys.forEach((key) => {
        this._deps[key] = new tracker.Dependency();
      });
    }
  }
  _markKeyChanged(key) {
    const genericKey = MongoObject.makeKeyGeneric(key);
    if (this._deps.hasOwnProperty(genericKey)) this._deps[genericKey].changed();
  }

I'd create a PR but I'm strugling to actually install a local version of simpl-schema to be able to test my change first.

--------Edit---------
So from some local testing it appears the issue is more to do with adding the tracker dependency for each sub schema key so that later marKeyChanged can find it. I have a fix created and will try get a PR created.